1. Field of the Invention
The present invention relates to an optical conversion relay amplification system for use as a relay amplification system to cover a dead spot of mobile communications, particularly relates to an optical conversion relay amplification system achievable of economic constitution avoiding a beat noise.
2. Description of the Related Art
Public mobile communications systems (hereafter referred to as mobile communications systems) such as car phone, portable phone system, PHS, paging system, etc. have a base station of the mobile communications (hereafter, base station) linked to a network of the higher rank than public networks, etc. (PSTN and ISDN) by a wire to allow the base station to communicate with mobile stations such as portable phones by wireless systems.
The communicable area (cover area) of the base station by wireless systems is exemplified by some kilometers to some ten kilometers in car phone and portable phone systems and some hundred meters in PHS. In addition, so-called xe2x80x9ccellular systemxe2x80x9d is applied to locate base stations in cover areas like cells in order to keep wider areas effectively using frequencies.
Further, to solve the problem of frequency resource according to an increased demand of radio waves, 1.5 GHz frequency band has been newly assigned to car/portable phone systems for use in addition to the conventional 800 MHz band and 1.9 GHz frequency band has been assigned to PHS for use. Furthermore, 2 GHz band is probably assigned to the portable phone of the next generation. All these band changes to high frequency bands have been carried out for mobile communications.
In these mobile communications systems using the cellular system, generally, wireless communications are done in high quality on outdoor ground, etc. However, even in a cover area of a base station, a radio wave difficultly reaches the inside of a tunnel, an underground shopping center, a basement of a building, and stories above ground of a big building, and thus, any auxiliary means are required to communicate with mobile wireless terminals in these sites by wireless systems.
These areas are named xe2x80x9cdead spotsxe2x80x9d. The dead spot generally appears in radio wave wireless communications not only of mobile communications systems, but also of such as official radios exemplified by police radio, fire radio, and train radio, TV broadcast, and radio broadcast.
As auxiliary means to allow wireless communications in these dead spots, an apparatus named xe2x80x9crelay amplification apparatus is generally usedxe2x80x9d.
The first antenna is located in a site capable of high quality wireless communications with a base station (a broadcast station in broadcasting) and the second antenna is located in a dead spot to amplify a high frequency signal arrived in the first antenna, transmit to the second antenna located in the dead spot by a wire, and radiate the high frequency signal amplified via the second antenna.
By such steps, downlink communications from the base station to the mobile station are kept. Besides, wireless communications in the dead spot become possible by that communications of a downlink line from the base station to the mobile station are established, and also by that in uplink communications from the mobile station to the base station, the high frequency signal radiated by the mobile station is caught by the second antenna to amplify and transmitted to the first antenna with a wire, and the amplified high frequency signal is radiated via the first antenna to output for transmission to the base station.
In addition, on the background of the popularized, wired, high quality communications networks of higher rank such as ISDN, there is a plan that a radio modem is equipped for the dead spot, e.g., underground shopping center where communications have a particularly large demand, directly connected to higher rank network such as ISDN by a wire.
Such radio modem modulates a high frequency signal by a signal inputted from the higher rank network, distributes the high frequency signal from the radio modem to an antenna (said second antenna) located in the dead spot, receives the frequency signal through the antenna, and demodulates the signal from the mobile station to output to the higher rank network.
The conventional relay amplification systems are specifically exemplified by a method in which in a dead spot such as tunnel, a leaky coaxial cable suitable to achieve a linear service area is used as the second antenna to link to the first antenna with a high frequency coaxial cable, and a method in which the second antenna is located in various sites to link to the first antenna with a high frequency coaxial cable.
However, a connection distance between the first antenna and the second antenna requires some kilometers on the situation. In this case, an attenuation (transmission loss) occurs as a problem in the high frequency coaxial cable. Particularly in latest years, frequencies assigned to mobile communications are moved to higher bands to require coaxial cables with a large diameter resulting in a problem of difficulty of laying construction.
Thus, to avoid transmission loss, the optical conversion relay amplification system has been proposed and partially practiced using analog optical fiber transmission technology in replacement to conventionally used the high frequency coaxial cable and leaky coaxial cable having a function as an antenna.
The technical situations of these optical conversions have been detailedly described in reference 1: Kuganuma et al., xe2x80x9cA tunnel booster 1.5 GHz digital mobile communicationsxe2x80x9d, NTT DoCoMo technical Journal, Vol. 2, No. 2 (1994), for example, for relay system, and also for example, reference 2: Sanada et al., xe2x80x9cAn optical transmission system for wireless base stationsxe2x80x9d, National Technical report, Vol. 39, No. 4 (1993) for optical transmission system to link to the system of a base station.
Besides, a detailed description has been given in the following general review about the application of transmission technology using an analog optical fiber for mobile communications: reference 3, Morita, K., et al., xe2x80x9cThe new generation of wireless communications based on fiber-radio technologiesxe2x80x9d, IEICE Trans. Comm., Vol. E76-B, No. 9 (1993), etc.
On the other hand, the following two main systems are conventional to accomplish these analog optical fiber transmission systems in a structure of optical transmission line: a star distribution connection used for point-to-multipoint connection to connect an optical transmitter/receiver of the first antenna side with an optical transmitter/receiver of the second antenna side for the downlink line from the base station of the mobile communications (simply, base station) with the mobile station (portable machine); on the contrary, transmission by connecting both optical transmitters/receivers as point-to-point connection in the uplink line from the portable machine to the mobile base station.
However, this connection system requires many optical fiber conductors. Therefore, A multibranching optical transmission system has been proposed to make the system economic by reducing the required optical fiber conductors.
This means that the number of optical fiber conductor is reduced to one for the downlink to distribute optical signals in a site requiring the distribution of the optical signal by using a point-to-two point optical branching device. The uplink, as well, transmits an optical signal from respective sites through a single optical fiber by multiplex using a point-to-two point optical multiplexer.
By this process, in the uplink, the optical fiber as transmission line is one in number and, in addition, an optical signal multiplexed by a single photo-detector is collectively received to make the scale of a photo-detector circuit small as an advantage.
As a specific example appeared in Japanese Published Unexamined Patent Application No.153255/94, a system has been described as that radio signals received by a plurality of antennaexe2x80x94corresponding to the second antenna/exe2x80x94put in a tunnel is converted to optical signals with different wavelengths keeping enough difference among E/O converters making pairs with respective antennae, respective optical signals are connected with optical couplers to transmit to objective sites using a single optical fiber to convert to electric signals using O/E converters for supplying to radio receiver.
In Japanese Published Unexamined Patent Application No.130322/97, a technology has been described that relay is carried out by connecting a ground relay station having the first antenna with a relay station in a dead spot having a leaky coaxial cablexe2x80x94corresponding to said second antennaxe2x80x94laid in a tunnel or an underground shopping center using an optical fiber.
As specific action, a radio signal from the base station of a mobile communication is received by a ground relay station via the first antenna, the high frequency electric signal thereof is converted to an optical signal to send via an optical fiber, the optical signal received by the relay station in the dead spot is converted to a high frequency electric signal and finally send through the leaky coaxial cable to the mobile station.
On the contrary, a radio signal sent from the mobile station is received by the leaky coaxial cable, the high frequency electric signal is converted to an optical signal in the relay station located in the dead spot to send via an optical fiber, the optical signal received by the ground relay station is converted to the high frequency electric signal to send to the base station of the mobile communications from the first antenna resulting in relay.
Particularly, as an example considering impossible area coverage caused by attenuation in a single leaky coaxial cable in a very long tunnel, etc., a technology has been described as that a plurality of the leaky coaxial cables is laid, a plurality of relay stations is established in the dead spot to service respective areas, an optical fiber is made as an optical transmission line of a single core multibranch type using a point-to-two points optical branching device/optical joint located near relay stations in respective dead spots.
As in said conventional examples 1 and 2, the use of the optical branching device/optical joint requires that branching ratio of the optical branching device in the downlink line is determined on situation in a system using a single optical fiber for an optical transmission line of the single core multibranch type.
On the other hand, in the uplink as like as the downlink, the multiplexing ratio of the optical multiplexer should be determined on situation. Besides, it should be considered that a beat noise caused by frequency difference among a plurality of optical signals multiplexed as electromagnetic waves should not affect the frequency band of the objective high frequency signal.
Said optical branching device and optical multiplexer can be made by using an optical coupler of fiber fusion type, for example. A single optical coupler can be used for both branching and multiplexing; a marketed optical coupler with branching ratio (multiplexing ratio) of each 5% class from 50%: 50% (power ratio) to 5% to 95% can be easily used.
Concerning methods to avoid a beat noise in the uplink, as shown in the first method of reference 4, it is the method proposed that a beat noise is detected positively to control to prevent abutting of emitting wavelengths (i.e., frequency) of a light emission diode to avoid the effect of the beat noise.
Reference 4: Taresawa et al., xe2x80x9ca single core, multibranching type optical fiber link for mobile communications employing automated frequency offset controlxe2x80x9d, IECIE Tech. Rep., RCS97- 70 (1994.)
However, in the first method, there are problems that more numbers of wavelengths cannot be used, light source for transmission is expensive, the optical multiplexing loss is high, and maintenance is difficult.
On the other hand, as the second method to avoid the beat noise, the following method has been proposed: no positive control of wavelength is carried out and the wavelength of a semiconductor laserxe2x80x94a light source of the optical signalxe2x80x94is selected to multiplex previously. According to the experiment of the inventors of the second method, a semiconductor laser of multilongitudinal mode oscillation, and in the optical signal of multipoint multiplexing using Fabry-Perot type laser as a light source of which condition of avoiding the beat noise is more strict in comparison with that of DFB (Distributed FeedBack) laser, a difference around 10 nm between central wavelengths is required to inhibit an effect on a frequency band of portable phones; the multiplex of 3-4 per single fiber is a limit in consideration of economic system, easy availability of the light source.
The reference 5 presents the proposal of application of said second method to the mobile communications systems.
Reference 5: Hukuie et al., xe2x80x9cConstitution of a multibranching optical transmission system for mobile the communicationsxe2x80x9d, 1996 IECIE Communications Society Meeting, B-492.
Avoiding the beat noise in the uplink is the most crucial technical problem in said high frequency signal transmission systems by analog optical modulation using the optical transmission lines of said multibranching and multinode multiplexing type.
Thus, Reference 6 proposed an optical multiaccess using a subcarrier relay node multiplexing system, in which any beat noise does not occur, as the third method to prevent a problem of optical beat noise caused by the optical multiplexing in the uplink.
Reference 6: Domon et al., xe2x80x9cA proposal of an optical multiaccess using the subcarrier relay node multiplexing systemxe2x80x9d, IECIE Tech. Rep., OCS93-99 (1994.)
The outline of said system is: an optical signal is once converted to electric signal in respective local nodes to multiplex electrically frequencies of new subcarriers followed by conversion to an optical signal to send to the optical transmission line.
The optical signal transmitted by multiplexing electrically the new subcarriers in respective local nodes one after another applying said method is received by an optical receiver in a center node to convert to an electric signal and a signal sent from a local node can be selected with a tuner etc.
As a specific example of said system, an optical network for relay and amplification using the optical multiaccess by the subcarrier relay node multiplexing system has been proposed in Japanese Published Unexamined Patent Application No. 191478/96. This system is herewith explained as a conventional optical conversion relay amplification system with reference to FIG. 8. The FIG. 8 is the diagrammatic illustration of the conventional optical conversion relay amplification system and the optical networks using the system.
The conventional optical conversion relay amplification system is, as shown in the FIG. 8, mainly composed of a central station 100, a mobile terminal station 103, an intermediate radio base station 102A and 102B, an end radio base station 102C, and a downlink optical fiber line 104A and an uplink optical fiber line 104B connecting the central station 100 and respective radio base stations 102.
In the central station 100, a downlink signal is modulated to send to the mobile terminal 103, converted to an optical signal to send to the downlink optical fiber line 104A; on the contrary, the uplink optical signal sent from the mobile terminal 103 to transmit is received from the uplink optical fiber line 104B to convert to a radio signal followed by demodulation.
The intermediate radio base station 102B is composed of an optical demultiplexer 121B demultiplexing the downlink optical signal from the downlink optical fiber line 104A, an optical receiver 123B converting the optical signal demultiplexed to radio signal, an amplifier 124B amplifying the radio signal, an antenna 135B sending the radio signal, an antenna 136B receiving the radio signal, an amplifier 132B amplifying the received signal, an optical receiver 125B converting the optical signal transmitted from a lower rank radio base station to a high frequency signal, a multiplexer 122B multiplexing the received signal amplified by the amplifier 132B with the high frequency signal from optical receiver 125B, and an optical transmitter 134B converting the multiplexed signal to an optical signal to send to the uplink optical fiber line 104B.
The intermediate radio base station 102A has the same constitution.
The end radio base station 102C has the constitution in which a function of demultiplexing optical signals in the downlink line and an optical relaying function in the uplink line have been removed from the constitution of the intermediate radio base station 102B, and is composed of an optical receiver 123C converting an optical signal from the downlink optical fiber line 104A to a radio signal, an amplifier 124C amplifying the radio signal, an antenna 135C sending the radio signal, an antenna 136C receiving the radio signal, an amplifier 132C amplifying the received signal, and an optical transmitter 134C converting the received amplified signal to an optical signal to send to the uplink optical fiber line 104B.
Further in the action of the conventional optical conversion relay amplification system, a downlink signal is converted to an optical signal in the central station 100 to send through the downlink optical fiber line 104A, partially demultiplexed by the demultiplexer 121B in the radio base station 102B (same in the radio base station 102A), converted to a radio signal by the optical receiver 123B, amplified by the amplifier 124B, and transmitted from the antenna 136B to the mobile terminal station 103B.
Furthermore, the optical signal transmitted from a lower rank radio base station (for example, radio base station 102C) to the uplink optical fiber line 104B is converted to a high frequency signal by the optical receiver 125B and inputted into the multiplexer 122B.
On the other hand, a radio signal sent from the mobile terminal 103 is received by the antenna 136B, amplified by the amplifier 132B, inputted into the multiplexer 122B, multiplexed with the high frequency arrived from said lower rank radio base station, subsequently, converted to an optical signal by the optical transmitter 134B and sent to the uplink optical fiber line 104B, and finally, converted to a radio signal by the central station 100.
Although the beat noise occurring in the uplink line can be avoided in said conventional optical conversion relay amplification system, there is a problem that the optical receiver having same performance is required by respective uplink and downlink lines resulting in an increased circuit size uneconomical.
The purpose of the present invention is to provide an optical conversion relay amplification system capable of a relay with the economical constitution in which the beat noise is avoided in uplink line of the optical transmission line of multibranching and multinode multiplexing type.
The present invention is an optical conversion relay amplification system, wherein said optical conversion relay amplification system has a fixed central station for transmission and receiving of a radio signal from a base station and a plurality of fixed relay stations located in intermediate position in the downlink optical fiber line from said fixed central station and an uplink optical fiber line to said fixed central station,
said fixed central station is a fixed central station converting a radio signal arrived from said base station to an optical signal to output to said downlink optical fiber line, and also converting the optical signal inputted from said uplink optical fiber line to a high frequency electric signal to send to said base station, and
said fixed relay station is a fixed relay station dividing the downlink optical signal from said downlink optical fiber line to multiplex with said uplink optical fiber line, converting the multiplexed optical signal to an electric signal, transmitting the downlink electric signal included in said converted electric signal to portable phone by wireless system, and also receiving the radio signal arrived from the said portable phone to multiplex with an uplink electric signal included in said converted electric signal and converting the multiplexed electric signal to an optical signal to output to said uplink optical fiber line.
Thus, the beat noise in the uplink line can be avoided and the communication of a portable phone located in a dead spot with a base station can be relayed in an economic constitution.
Further, the present invention has a fixed central station working for transmission/receiving of a signal to/from public networks, and a plurality of fixed relay station located in intermediate position of a downlink optical fiber line from said fixed central station and an uplink optical fiber line toward said fixed central station, in an optical conversion relay amplification system, wherein
said fixed central station is a fixed central station modulating a signal received from said public network to make a high frequency signal to convert to an optical signal for output to said downlink optical fiber line, and also converting the optical signal inputted from said uplink optical fiber line to a high frequency electric signal, demodulate to transmit to said public network, and
said fixed relay station is a fixed relay station branching the downlink optical signal from said downlink optical fiber line to multiplex with said downlink optical fiber line, converting the multiplexed optical signal to an electric signal, transmitting downlink electric signal contained in the converted electric signals to a portable phone by radio, and also receiving an electric signals from said portable phone to multiplex with uplink electric signals contained in said converted electric signals, and converting the multiplexed electric signals to optical signals to output to said uplink optical fiber line.
Thus, communications between a portable phone located in a dead spot and a public network can be relayed with the economical constitution in which the beat noise is avoided in uplink line.
Furthermore, the present invention has a fixed central station to receive a radio signal arrived from a base station and a plurality of fixed relay stations located in an intermediate position in downlink optical fiber line from said fixed central station and an uplink optical fiber line to said fixed central station in an optical conversion relay amplification system, wherein
said fixed central station is a fixed central station, wherein a radio signal received from said base station is converted to an optical signal to output to said downlink optical fiber line, and also an optical signal inputted from said uplink optical fiber line is converted to an electric signal to receive a surveillance signal of a surveillance result in said fixed relay station, and
said fixed relay station is a fixed relay station, wherein a downlink optical signal branches from said downlink optical fiber line to multiplex with said uplink optical fiber line, the multiplexed optical signal is converted to an electric signal, a downlink electric signal contained in the converted electric signals is transmitted to a radio paging receiver by wireless system, an uplink electric signal contained in said converted electric signals is multiplexed with the surveillance signal, and the multiplexed electric signals are converted to optical signals to output to said uplink optical fiber line.
Thus, paging from a base station to a paging receiver located in a dead spot can be relayed with the economical constitution in which the beat noise is avoided in the uplink line, and also a surveillance signal in a fixed relay station can be transmitted to the fixed central station by relaying.
Further, the present invention has a fixed central station to receive a signal from a public network and a plurality of fixed relay station located in intermediate position of a downlink optical fiber line from said fixed central station and an uplink optical fiber line to said fixed central station in an optical conversion relay amplification system,
said fixed central station is a fixed central station, wherein a signal received from said public network is modulated to a high frequency signal, converted to an optical signal to output to said downlink optical fiber line, and also the optical signal inputted from said uplink optical fiber line is converted to an electric signal to receive a surveillance signal of a surveillance result in said fixed relay station,
said fixed relay station is a fixed relay station, wherein a downlink optical signal branches from said downlink optical fiber line to multiplex with said uplink optical fiber line, the multiplexed optical signal is converted to an electric signal, a downlink electric signal contained in the converted electric signals is transmitted to a paging receiver by wireless system, an uplink electric signal contained in said converted electric signals is multiplexed with the surveillance signal, the multiplexed electric signals are converted to optical signals to output to said uplink optical fiber line.
Thus, paging from a public network to a paging receiver located in a dead spot can be relayed with the economical constitution in which the beat noise is avoided in uplink line, and also a surveillance signal in a fixed relay station can be transmitted to the fixed central station by relaying.
Furthermore, the present invention has a fixed central station to transmit/receive a radio signal to/from a base station and a plurality of fixed relay station located in intermediate position of respective downlink optical fiber line from said fixed central station and uplink optical fiber line to said fixed central station in an optical conversion relay amplification system,
said fixed central station is a fixed central station, wherein a surveillance controlling signal controlling surveillance in said fixed relay station is multiplexed with a radio signal received from said base station, said multiplexed electric signal is converted to an optical signal to output to said downlink optical fiber line, and also an optical signal inputted from said uplink optical fiber line is converted to an electric signal, and said electric signal is separated into a signal to said base station and a surveillance signal of a surveillance result in said fixed relay station to transmit said signal for said base station to said base station, and also said surveillance signal is received, and
said fixed relay station is a fixed relay station, wherein a downlink optical signal branches from said downlink optical fiber line to multiplex with said uplink optical fiber line, the multiplexed optical signal is converted to an electric signal, a downlink electric signal contained in the converted electric signals is separated into a signal for a portable phone and a surveillance controlling signal, the separated signal for a portable phone is transmitted to a portable phone by wireless system, and also surveillance is carried out on the basis of said separated surveillance controlling signal, the surveillance signal yielded by said surveillance, the radio signal received from said portable phone, and an uplink electric signal contained in said converted electric signals is multiplexed, and the multiplexed electric signals are converted to optical signals to output to said uplink optical fiber line.
Thus, communications between a portable phone and a base station located in a dead spot can be relayed with the economical constitution in which the beat noise is avoided in uplink line, and also communications of a surveillance controlling signal and a surveillance signal between a fixed central station and a fixed relay station can be relayed.
Also furthermore, the present invention has a fixed central station to transmit/receive a signal to/from a public network and a plurality of fixed relay station located in intermediate position of respective downlink optical fiber line from said fixed central station and uplink optical fiber line to said fixed central station in an optical conversion relay amplification system,
said fixed central station is a fixed central station, wherein a surveillance controlling signal controlling surveillance in said fixed relay station is multiplexed with a high frequency signal generated by modulating a signal received from said public network, said multiplexed electric signal is modulated and converted to an optical signal to output to said downlink optical fiber line, and also an optical signal inputted from said uplink optical fiber line is converted to an electric signal, and said electric signal is separated into a signal for said public network and a surveillance signal of a surveillance result in said fixed relay station to transmit a signal for said public network to said public network after demodulation, and also said surveillance signal is received, and
said fixed relay station is a fixed relay station, wherein a downlink optical signal branches from said downlink optical fiber line to multiplex with said uplink optical fiber line, the multiplexed optical signal is converted to an electric signal, a downlink electric signal contained in the converted electric signals is separated into a signal for a portable phone and a surveillance controlling signal, the separated signal for a portable phone is transmitted to a portable phone by wireless system, and also surveillance is carried out on the basis of said separated surveillance controlling signal, the surveillance signal yielded by said surveillance, the radio signal received from said portable phone, a surveillance signal outputted from said surveillance controlling signal unit, and an uplink electric signal contained in said converted electric signals are multiplexed, and the multiplexed electric signals are converted to optical signals to output to said uplink optical fiber line.
Thus, communications between a portable phone and a public network located in a dead spot can be relayed with the economical constitution in which the beat noise is avoided in uplink line, and also communications of a surveillance controlling signal and a surveillance signal between a fixed central station and a fixed relay station can be relayed.
Finally, in an optical conversion relay amplification system of the present invention, the antenna of the relay station is a leaky coaxial cable and with the economical constitution in which the beat noise is avoided in uplink line, and also communications of a portable phone or a paging receiver with a base station of mobile communications or a higher rank network can be relayed for linearly extending dead spots as service areas.